This invention is in the area of polymers for controlled delivery of substances, and is specifically a polyanhydride prepared from oligomerized unsaturated aliphatic acids.
There has been extensive research in the area of biodegradable controlled release systems for bioactive compounds. Biodegradable, biocompatible matrices for drug delivery are useful because they obviate the need to remove the drug-depleted device.
The most desirable polymeric matrix for drug delivery is one that is hydrophobic, stable, strong, flexible, soluble in organic solution, has a low melting point, and degrades linearly over time. The polymer must be hydrophobic so that it retains its integrity for a sufficient time when placed in an aqueous environment, such as the body, to effect controlled release. The polymer must be stable to storage for an extended period before use. The polymer must be strong, yet flexible enough that it does not crumble or fragment during use.
Controlled release devices are typically prepared in one of several ways. In one method, the polymer is melted, mixed with the substance to be delivered, and then cooled. Melt fabrication requires that the polymer have a melting point that is below the temperature at which the substance to be delivered and polymer degrade or become reactive. Alternatively, the device can be prepared by solvent casting, in which the polymer is dissolved in a solvent, and then the substance to be delivered is dissolved or dispersed in the solution. The solvent is then evaporated, leaving the substance in the polymeric matrix. Solvent casting requires that the polymer be soluble in organic solvents.
For controlled drug delivery, the polymer must degrade by surface erosion and not by bulk erosion. Surface erosion occurs when the rate of hydrolytic degradation on the surface of the polymeric structure is faster than the rate of degradation in the interior of the polymeric structure. Bulk erosion occurs when the polymer incorporates water into the center of the matrix, rendering the polymer sponge-like with hole, or channels in the matrix. When bulk erosion occurs, the material to be delivered is released through the channels in a rapid, uncontrolled fashion.
Many polymers have been evaluated for use as controlled drug delivery matrices, including polyesters, polyamides, polyurethanes, polyorthoesters, polyacrylonitriles, and polyphosphazenes. None of these polymers have exhibited the desired combination of characteristics for use in the controlled delivery of substances.
Polyanhydrides have also been studied for use in controlled delivery devices. See, for example, U.S. Pat. No. 4,891,225 to Langer, et al., U.S. Pat. No. 4,886,870 to D'Amore, et al., Leong, et al., J. Med. Biomed. Mater. Res. 19, 941 (1985); and Leong, et al., J. Med. Biomed. Mater. Res. 20, 51 (1986). One of the first polyanhydrides studied with respect to controlled release characteristics was poly(bis(p-carboxyphenoxy)methane anhydride), as described by Rosen, et al., Biomaterials 4, 131 (1983). The aromatic polyanhydride exhibited near zero order (linear) erosion and release kinetics at 37.degree. C. and 60.degree. C., in vitro.
Shortly thereafter, three related polyanhydrides, poly 1,3-(bis(p-carbophenoxy)propane anhydride (p-CPP) (an aromatic polyanhydride), the polymer formed from the copolymerization of CPP with sebacic acid (a copolymer of an aromatic diacid and an aliphatic diacid), and polyterephthalic acid (an aromatic anhydride) were prepared and studied, as described by Leong, et al., J. Med. Biomed. Mater. Res. 19, 941 (1985).
It was found that aromatic polyanhydrides have unacceptably long degradation rates. For example, it was estimated that a delivery device prepared from p-CPP would require more than three years to completely degrade in vivo. Further, anhydride homopolymers of aromatic or linear aliphatic dicarboxylic acids were found to be highly crystalline and have poor film forming properties. Aromatic polyanhydrides also have high melting points and low solubility in organic solvents.
As described in U.S. Pat. No. 4,757,128 to Domb and Langer, high molecular weight copolymers of aliphatic dicarboxylic acids with aromatic diacids are less crystalline than aromatic or linear aliphatic polyanhydrides, and they form flexible films.
Degradation rates are also increased by copolymerizing an aromatic dicarboxylic acid with an aliphatic diacid; however, bulk erosion occurs because areas of the polymer containing aliphatic anhydride linkages erode faster than aromatic anhydride linkages, leaving channels in the matrix through which the substance to be delivered is released in an uncontrolled fashion. For example, in the p-CPP sebacic acid copolymer, the aliphatic anhydride bonds are cleaved and all drug released in ten days, while the aromatic regions remained intact for over five months. Further, the copolymers have inferior mechanical properties; they become brittle and crumble into flakes on exposure to moisture.
Polymers prepared from linear aliphatic diacids are hydrophilic solids that degrade by bulk erosion, resulting in a rapid release of the drug from the polymeric matrix. Hydrophobicity is increased by copolymerizing the linear aliphatic diacids with aromatic diacids, however this approach results in an increase in the polymer melting temperature and a decrease in organic solvent solubility. Furthermore, it does not improve the drug release profile but increases the degradation and the elimination time of the polymer both in vivo and in vitro. Since both homopolymers and copolymers of linear aliphatic diacids are very sensitive to moisture, they require extremely anhydrous and low temperature storage conditions.
Several attempts have been made to improve the controlled release characteristics of polyanhydrides by altering the method of synthesis or the molecular weight. See, for example, U.S. Pat. No. 4,857,311 to Domb and Langer, describing polyanhydrides with a uniform distribution of aliphatic and aromatic residues in the chain, prepared by polymerizing a dicarboxylic acid with an aromatic end and an aliphatic end; U.S. Pat. No. 4,888,176 to Langer, et al., describing the preparation of high molecular weight polyanhydride controlled delivery devices; and U.S. Pat. No. 4,789,724 to Domb and Langer describing the preparation of very pure anhydride copolymers of aromatic and aliphatic diacids.
There remains is a strong need for a polymer having the desired characteristics of hydrophobicity, stability, strength, flexibility, organic solubility, low melting point, and appropriate degradation profile, for use as the matrix for a controlled delivery device.
It is therefore an object of the present invention to provide a biodegradable polymer that is highly hydrophobic, and that degrades by surface erosion to release an incorporated substance in a controlled manner.
It is another object of the present invention to provide a polymer that is thermodynamically and hydrolytically stable and that requires mild storage conditions.
It is yet another object of the present invention to provide a strong and flexible polymer film that degrades in vivo into a soft material that can be gradually eliminated from the body.
It is a further object of the present invention to provide a controlled delivery device that is suitable for intraperitoneal implantation.